1. Field of the Invention
The present invention relates to a ceramic honeycomb structure for use in a filter for purifying an exhaust gas of an internal combustion engine, represented by a diesel particulate filter (DPF), and a method of manufacturing the structure.
2. Description of the Related Art
Since a large amount of particulates (particulate substances) are contained in an exhaust gas discharged from an internal combustion engine such as a diesel engine, and contain as a main component carbon that is a cause for environmental pollution, a filter for trapping the particulates is sometimes disposed in an exhaust system of the engine.
In general, in the filter to be used for such purpose, as shown in FIGS. 1(a) and (b), there is used a ceramic honeycomb structure having a plurality of cells (through channels) 9 partitioned by porous partition walls 7 and extending through the structure in an axial direction. One end portion of each predetermined cell 9a is plugged with a plugging portion 11 constituted of a plugging material with which the cell is filled. The other end portion of each remaining cell 9b is similarly plugged with the plugging portion 11 on a side opposite to the end portion of the predetermined cell 9a. 
After the exhaust gas flows into the filter from one end surface 3 of the filter constituted of such honeycomb structure, and the particulates and the like contained in the gas are removed, the gas flows out of the other end surface 5. Specifically, first, the exhaust gas flows into the cell 9b whose end portion is not plugged in the end surface 3 on an inflow side of this filter and whose end portion is plugged in the end surface 5 on an outflow side. The gas passes through the porous partition wall 7 to move to the cell 9a whose end portion is plugged in the inflow-side end surface 3 and whose end portion is not plugged in the outflow-side end surface 5, and is discharged from the cell 9a. Moreover, in this case, the partition wall 7 constitutes a filtering layer, and the particulates in the gas are trapped and accumulated on the partition walls 7.
In recent years, there has been proposed such ceramic honeycomb structure in which a porosity of each plugging portion is set to be higher than that of each partition wall which partitions the cell (see, e.g., Japanese Patent Application Kokai No. 7-332064). When the porosity of the plugging portion is set to be higher than that of the partition wall, a pressure loss drops. In addition, when a heat capacity of the plugging portion is reduced, a fast heating property and a thermal shock resistance are improved. In a case where the structure is used in a diesel particulates filter (DPF) for trapping the particulates discharged from the diesel engine, a regeneration treatment is performed to heat the filter periodically and burn and remove the particulates accumulated in the filter. In this case, there is an effect that a temperature of the filter rises fast, and a treatment time can be reduced.
However, when the porosity of the plugging portion is set to be higher than that of the partition wall as described above, a bonding strength between the partition wall and the plugging portion drops. Therefore, there is a possibility that the plugging portion falls owing to vibration, exhaust pressure, and the like during a long-time use. Therefore, there is actually a restriction on enhancing of the porosity of the plugging portion, and any sufficient effect is not obtained in the present circumstances. In a case where the porosity of the partition wall is high, a partition wall strength is low. Therefore, there sometime occurs destruction of the partition wall due to a thermal shock in the plugging portion and in the vicinity of the portion.